Neutron tubes of this kind are used in techniques for the examination of substances by means of fast, thermal, epithermal or cold neutrons neutronography, analysis by activation, analysis by spectrometry of inelastic diffusions or radiative captures, diffusion of neutrons, etc.
In order to make these nuclear techniques as effective as possible, longer tube service lives are required for the corresponding emission levels.
The fusion reaction d(3.sub.H' 4.sub.He)n which supplies 14 MeV neutrons is most commonly used because of its large effective cross-section for comparatively low ion energies. However, regardless of the reaction used, the number of neutrons obtained per unit of charge in the beam always increases in proportion to the increase of energy of the ions directed towards a thick target, that is to say mainly beyond ion energies obtained in the sealed tubes available at present and which are powered by a high voltage not exceeding 250 kV.
Erosion of the target by ion bombardment is one of the principal factors restricting the service life of a neutron tube.
The erosion is a function of the chemical nature and the structure of the target, on the one hand, and of the energy of the incident ions and their density distribution profile on the surface of impact, on the other hand.
In most cases the target is formed by a hydride (titanium, scandium, zirconium, erbium, etc.) which hydride is capable of binding and releasing large quantities of hydrogen without substantially affecting its mechanical strength. The total quantity bound is a function of the temperature of the target and of the hydrogen pressure in the tube. The target materials used are deposited in the form of thin layers whose thickness is limited by the problems imposed by the adherence of the layer to its substrate. One way of retarding the erosion of the target, for example, is to construct the absorbing active layer as a stack of identical layers which are isolated from one another by a diffusion barrier. The thickness of each of the active layers is in the order of magnitude of the penetration depth of deuterium ions striking the target.
Another method of protecting the target, thus increasing the service life of the tube, consists in the influencing of the ion beam so as to improve its density distribution profile on the surface of impact. For a constant total ion current on the target electrode, leading to a constant neutron emission, this improvement will result from an as uniform as possible distribution of the current density across the entire target surface exposed to the ion bombardment.
One of the ways of reducing this maximum density is to use the divergence of the beam in the space between the point of convergence and the target. In this space any increase of the path of the ions by a factor x is translated into a reduction of the type 1/x.sup.2 of the maximum bombardment density.
In a sealed neutron tube the pressure of the deuterium-tritium mixture necessary for obtaining the ion current is of primary importance and is the same throughout the tube. Therefore, the ions extracted and accelerated toward the target will react with the gas molecules in order to produce ionisation effects, dissociation effects and charge exchange effects. This results, on the one hand, in a lower mean energy of the ions on the target, that is to say a reduction of the production of neutrons, and, on the other hand, in the formation of ions and electrons which are subsequently accelerated and bombard the ion source or the electrodes of the tube.
This results in energy spots which increase the temperature of electrode materials such as molybdenum or stainless steel or pyrolytic carbon. The heating of these materials causes desorption of impurities such as carbon oxide enclosed in the neutron tube, thus reducing the quality of the tube atmosphere. The ions of impurities formed in the tube, for example Co.sup.+, bombard the target with a pulverisation coefficient which is a factor from 10.sup.2 to 10.sup.3 higher than that of the deuterium-tritium ions, thus causing an substantial increase of the erosion.
These effects increase as the operating pressure is higher and the ion path is longer. Thus, a correction of the inhomogeneities of the bombardment of the target which could be realised by increasing the ion path is ineffective because of the increase of the ion-gas reactions which is greater than or equal to a simple proportionality factor.